Ignore invalid host header between go1.6 and old docker clients

BenchmarkWithHack-4	   50000	     37082 ns/op	  44.50
MB/s	    1920 B/op	      30 allocs/op
BenchmarkNoHack-4  	   50000	     30829 ns/op	  53.52
MB/s	       0 B/op	       0 allocs/op

Signed-off-by: Brian Goff <cpuguy83@gmail.com>
Signed-off-by: Antonio Murdaca <runcom@redhat.com>

cmd/dockerd/hack/malformed_host_override.go

@@ -0,0 +1,116 @@
+// +build !windows
+
+package hack
+
+import "net"
+
+// MalformedHostHeaderOverride is a wrapper to be able
+// to overcome the 400 Bad request coming from old docker
+// clients that send an invalid Host header.
+type MalformedHostHeaderOverride struct {
+	net.Listener
+}
+
+// MalformedHostHeaderOverrideConn wraps the underlying unix
+// connection and keeps track of the first read from http.Server
+// which just reads the headers.
+type MalformedHostHeaderOverrideConn struct {
+	net.Conn
+	first bool
+}
+
+var closeConnHeader = []byte("\r\nConnection: close\r")
+
+// Read reads the first *read* request from http.Server to inspect
+// the Host header. If the Host starts with / then we're talking to
+// an old docker client which send an invalid Host header. To not
+// error out in http.Server we rewrite the first bytes of the request
+// to sanitize the Host header itself.
+// In case we're not dealing with old docker clients the data is just passed
+// to the server w/o modification.
+func (l *MalformedHostHeaderOverrideConn) Read(b []byte) (n int, err error) {
+	// http.Server uses a 4k buffer
+	if l.first && len(b) == 4096 {
+		// This keeps track of the first read from http.Server which just reads
+		// the headers
+		l.first = false
+		// The first read of the connection by http.Server is done limited to
+		// DefaultMaxHeaderBytes (usually 1 << 20) + 4096.
+		// Here we do the first read which gets us all the http headers to
+		// be inspected and modified below.
+		c, err := l.Conn.Read(b)
+		if err != nil {
+			return c, err
+		}
+
+		var (
+			start, end    int
+			firstLineFeed = -1
+			buf           []byte
+		)
+		for i, bb := range b[:c] {
+			if bb == '\n' && firstLineFeed == -1 {
+				firstLineFeed = i
+			}
+			if bb != '\n' {
+				continue
+			}
+			if b[i+1] != 'H' {
+				continue
+			}
+			if b[i+2] != 'o' {
+				continue
+			}
+			if b[i+3] != 's' {
+				continue
+			}
+			if b[i+4] != 't' {
+				continue
+			}
+			if b[i+5] != ':' {
+				continue
+			}
+			if b[i+6] != ' ' {
+				continue
+			}
+			if b[i+7] != '/' {
+				continue
+			}
+			// ensure clients other than the docker clients do not get this hack
+			if i != firstLineFeed {
+				return c, nil
+			}
+			start = i + 7
+			// now find where the value ends
+			for ii, bbb := range b[start:c] {
+				if bbb == '\n' {
+					end = start + ii
+					break
+				}
+			}
+			buf = make([]byte, 0, c+len(closeConnHeader)-(end-start))
+			// strip the value of the host header and
+			// inject `Connection: close` to ensure we don't reuse this connection
+			buf = append(buf, b[:start]...)
+			buf = append(buf, closeConnHeader...)
+			buf = append(buf, b[end:c]...)
+			copy(b, buf)
+			break
+		}
+		if len(buf) == 0 {
+			return c, nil
+		}
+		return len(buf), nil
+	}
+	return l.Conn.Read(b)
+}
+
+// Accept makes the listener accepts connections and wraps the connection
+// in a MalformedHostHeaderOverrideConn initilizing first to true.
+func (l *MalformedHostHeaderOverride) Accept() (net.Conn, error) {
+	c, err := l.Listener.Accept()
+	if err != nil {
+		return c, err
+	}
+	return &MalformedHostHeaderOverrideConn{c, true}, nil
+}